Chrome tanning solutions and method of making and using the same



Paten 1.4

6 AND usmo 'r s Hiram B, Channon, Portsmouth, VI" nor to Virginia smelting Company, West Norfolk, Via, a corporation of Maine No Drawing. Application Decemmr 29, 1942,

Scrial'No. 4705470 e Claims. ('Ci. s- -sast) This invention relates to combination chrome tag solutions and to the procedure of preparing and using the same.

In the preparation of tanning liquors forthe tag of leather by the procedures of chrome tannage as heretofore developed, it has been the common practice to use the aqueous solution of sodium or potassium dichromate which is reduced by the addition of sulphur dioxide or of sugar or like reducing agents.

Such reduction of dichromate is in general satisfactory, but it is susceptible to numerous conditions and variations and the resulting tannage liquor and its tanning activity vary accordingly. The concentrations of the several reagents, the rates and temperatures at which they are reacted, and their ultimate proportions are all important features. In all cases an excess of sugar is required to effect complete reduction of the dichromate, so that various by-products are formed. Moreover, too rapid a reaction will caramelize or carbonize a part of the sugar, which is detrimental I to its use as a tanning agent, and the reducing reaction proceeds out of control, with unknown variations in the composition of the end prod-. ucts accordingly.

It is, therefore, an object of this invention to provide improved tannage solutions or liquors for the chrome tanning of leather which may be definitely prepared of reliable composition and which may be used in the tanning procedure with uniformity of operation and dependability of the results to be obtained. Other objects will appear from the following disclosure.

In accordance with the present invention it is discovered that a properly reduced chrome tanhing liquor may be accurately and definitely prepared and that it will be eflectively maintained in a proper condition for functioning dependably throughout the tanning operation by the preliminary addition thereto-before applyin to the hides to be tanned-of certain characteristically strong organic acids.

Organic'aclds are in many cases too weakly acidic, though the relatively strong organic acids such as formic; oxalic and acetic acids are effective for the purpose of this invention, and more especially so when used in conjuction with each other in a given reduced chrome tanning liquor. A plurality of the organic acids may be employed, therefore, with advantageous results in getting a more complete and greatly improved tonnage.

Chrome tanning liquors, as thus prepared, manifest a more uniform tanning action from the start and carry out automatically 9. more uniform and complete tannase hide and resulting leather than any of the usually reduced chrome tan liquors as mown and used in the tanning art before the present invention,

6 whether reduced from the dichromate by sulphur The combined tanning action of the ingredients of such treated iiquors, containing the reduced chrome and the organic acid thus added in accordance with the present invention, is characterized by a gradual, uniform, progressiveand com plete permeation of the hide to a uniform concentration throughout, before the tanning action is eiiective to retard or prevent such permeation. Hence, as the tanning action proceeds, the hide is developed into leather as a whole, and the inner portions are tanned as well and as completely as the outer portions. Moreover, the fiber elements of the hide structure are individually penetrated and their lumens filled as well as the porous spaces and the hide substances which are between 30 the fibers. This is an extremely importantfeature of the present invention. And yet it is secured without increased time being required for the tanning operation and without the necessity of expensive reagents or additional equipment.

It is thus discovered that by tanning hides with the combination tanning solutions as thus prepared an improved tanning operation or function is accomplished in the process and that a fundamental improvement in the tanning characteris tics of the finished leather is obtained, as will be pointed out more iully below.

Moreover, if these conditions are observed, it is. notjnecessary to reduce the dichromate with sugar. Therefore, the dichromate solution may 5 be more definitely reduced with sulphur dioxide and then stabilized by the addition of the weakly acidic reagent, without introducing the variable by-products of the sugar reaction.

, For example, in a given case, 100 lbs. of sodium dichromate were dissolved in gallons of water throughout the untannedstantially theoretical proportion cording to the reaction:

. The solution which has been reduced as above described may be treated with one or more organic acids, so chosen and in such amounts as to give the resulting treated liquor a pH value between 2.3 and 3.0. For example, a mixture of 2% (64.5 lbs.) acof oxalic acid, 1% formic acid and 1% acetic 1 The solution, as thus prepared and in the quantities and concentrations indicated, was found to be satisfactory for direct use in the tanning of 1000 lbs. of hides weighed after pickling and draining.

In another mode of procedure 1000 lbs. of dichromate were dissolved in 250gallons of water, lbs. of acetic acid (100%) were added to the resulting solution, and liquid sulphur dioxide introduced gradually until the temperature reached 120 F. The sulphur dioxide stream was then shut off and 100 lbs. ofcaustic soda, dissolved in about 50 gallons of water, added. The stream of sulphur dioxide was then turned on and continued until reduction of the solution was complete. Then added 10 lbs. formic acid (100%) and lbs. of oxalicacid (100%). Agitation was continued till the additions were thoroughly dissolved and the volume increased by additions of water to provide a concentration corresponding to 2 lbs. of original dichromate to the gallon, or 500 gallons total final volume.

The addition of the acetic acid tends to give a bluish cast to the chrome, but does not take part in the reduction. It could be added last, i. e., with the oxalic and formic acids if desired.

It is to be observed that in the combination tanning liquors as thus prepared the weakly acid reagent, whether provided in the form of a strong organic acid or acids such as acetic formic and oxalic acids combined, is present in a molecular ratio with respect to the sodium dichromate from which the tanning liquor has been prepared of suitable for use as upper leather.

one-sixth to one-third. Lesser proportions of the acidic reagent are in general ineffective while larger proportions are, characterized'by a tendency to convert the chrome content of the liquor I from the green form or modification to the violet or reddish modification, which is not susceptible of control ofthe tanning action as herein discovered, disclosed and claimed.

The combined tannage action of the chrome liquor as thus modified by the acidic reagent is different both in kind and in degree from chrome tan liquors of the prior art, and also in respect of the tanning function and of the tanned leather product obtained, as will be pointed out and discussed below.

In the tanning procedure with such chrome tannage liquors, 1000 lbs. of stock may be taken,

after pickling and draining, and laid down with gallons of water in which 40 lbs. of common salt have been dissolved. The charge as thus.

prepared is handled in a wheel or rotatable drum, to which the reduced dichromate solution, asobtained by any of the procedures above described, is added. Thus, atotal quantity of reduced chrome solution, under I, corresponding to 40 lbs. of dichromate (in a concentration of 2 lbs. to the gallon and hence equal to 20 gallons) are diluted with water to 30 gallons. Thus, for example, on the'weight of the dichromate it may contain Per cent Oxalic acid 2 to 4 Formic acid l to 2 Acetic acid l to 2 1 and complete solution of all efiected.

This solution is then added to the wheel in 4 equal portions, one hour apart, and the drumming of the hides continued for 3 hours thereafter.

Then 1 4% of the stock) is dissolved in water to a concentration of lb. per gallon (or 12% lbs. of biof the reagents is carbonate in 50 gallons of solution) and this is two equal portions, 15

is more especially In making leather for other purposes, the manipulation, concentration, periods of treatment and amounts of the acidic reagent present and -of the sodium bicarbonate subsequently added will be adapted accordingly, in ways known to or readily determlnable by those skilled in the art from the above description and examples.

It is also found that acid solutions of dichromate-or chromic acid solutions which have been reduced with sugar or other organic reducing agents while undergoing indeterminate amounts of side reactions and containing variable amounts of by-products accordingly may be treated by the addition of the above described strong organic acid reagents thereto, in accordance with the invention.

During the latter part of the tanning action thepH value of the tanning liquor usually rises from about 3.0 to 4.6. But during the preliminary part of the tanning action the pH value of the combination tanning liquor prepared as herein described is such as'to serve the double function of keeping the tanning agents in solution and of definitely promoting its penetration and permeation of the several structures of the leather substance as a whole and more especially of the individual fibers, as will be described below.

It is found that leathers tanned by the present invention have a modified feel and resiliency in contrast to those obtained by a chrome tanning liquor reduced by mineral reducing agents only. Moreover, whereas ordinary pickled stock shows a tendency to curl at 55 C., the products obtained as above described may be boiled without curling. These products are full and resilient,

giving the bodyand resistance to bending and sodium bicarbonate (on the weight tested for by folding lubricates the at etc. There is, accordingly, an interchange of fluids through the cell walls of these lumens and discharge of metabolized products back from the lumens into the lymphatic system, for example. In the various treatments to which hides are subjected, the tissue fiuids contained in the lumens of the fibers are substantially removed,

thus leaving the hollow fibers filled with water only and upon its removal, as by.drying, the fibers collapse and the hide as a whole loses its resiliency and is either still and hard when dry or soft and flaccid if moist.

In tanning hides, therefore, it is desirable to impart resiliency to these individual fibers, as such, as well as to the spaces or materials between them. This is not eflected by chrome tanning liquors and processes of the prior art. In order to do so the interior portions of the lumens of the individual fibers must be treated, at least partially, with the tanning liquor. To accomplish this result, it is necessary for the tanning solution to permeate both the leather structure and the walls of the individual fibers, and thus enter the lumens laterally throughout their length as well as to enterthe lumens at their ends or other openings, and thus penetrate them longitudinally from one end to the other, by osmosis and capillarity.

It is found that the combinationdichromate tanning solutions which have been prepared as also more resilient above described are efiective not only to pass through the general structure of the hide interly but also to penetrate the walls into the lumens of the individual fibers intrally, and even to permeate the walls of such individual fibers, and thus to enter the lumens of such fibers and to fill both the lumens and the fiber walls with the tanning solution. This noticeably expands and plumps out each individual fiber in the leather. In the tanning action, effected by the present invention, the hydrated chromium component of the impregnating liquor is separated out and being already within the lumens precipitates upon the inner walls of the lumens of the fibers and .thus fills or at least coats and partially fills them with an initially gelatinous precipitate which may, uponsubsequent extreme drying, as in old leather obiects made therefrom, be regroup consisting of oxalic,

fibers are therefore relieved, by the more or less filled inner structure of the lumens, when the leather is folded, so that the fibers are not so sharply folded and creased as they would be if the lumens were collapsed and the inner surfaces were either adherent to each other or frictionaliy in contact with each other.

The result of tanning hides with the combination chrome tanning solutions prepared in accordance with the present invention is that the fib'ers are individually tanned on the inside and piumped. Therefore, they are more flexible and to return to their natural shape and disposition, and hence produce leather of greater body and fullness than is obtained by tanning liquors which are prepared in accordance with the prior art and practices.

This application is acontinuation-in-part of my copending application Serial No. 264,189, filed March 25, 1939.

I claim:

1. A combination chrome tannage for hides. comprising the steps of treating a sulphur dioxide reduced dichromate tanning liquor with an organic acid selected from'the group consisting of oxalic, formic and acetic acids and mixtures thereof, in a total molecular proportion of onesixth to one-third of the dichromate to a pH value between approximately 2.8 and 3.0, and then applying the resulting liquor to the hides to be tanned, whereupon the liquor plumps the hides and penetrates the fiber structure and lumens of the hides, preliminary to the tanning action thereof.

2. A chrome tanning liquor consisting substantially of a solution of mono-basic chromic sulphate, and an organic acid selected from the formic and acetic acids and mixtures thereof in the molecular ratio of approximately one-sixth to one-third of the mono-basic chromic sulphate present, having a duced to a finely divided powder. In the gelatand position after folding, than a collapsed and stiffened fiber can do. 'lhey, therefore, impart resiliency and life to the Moreover, since they are to an appreciable degree kept moist by the gelatinous precipitate thus deposited in them, such fibers remain pliable longer than would otherwise be the case. And if and when they may ultimately dry out and iose'their moisture, the residual finely divided chromium hydroxide or oxide,which is then left, its walls oi the lumen, one from the other, and keeps them apart. The

pH value between 2.8 and 3.0 whereby, upon contact with pickled hides, the liquor plumps the hides and penetrates the fiber structure and lumens thereof, preliminary to the tanning action.

3. Method of preparing a. combination chrome tanning liquor, preliminary to the operation oi tanning pickled and drained hides therewith, comprising the steps of reducing an aqueous solution of an alkali metal dichromate withsulphur dioxide, while maintained in acidic condition. completely to the form of mono-basic chromic sulphate, and then treating the latter with an organic acid selected from the group consisting also exert their elasticity more effectively to resume their original shape of oxalic, formic and acetic acids and mixtures thereof in the molecular proportion of approximately one-simh to one-third of the dichromate to a pH value between approximately 2.8 and 3.0.

, whereby, upon contact with the pickled and drained hides, the liquor plumps the hides and Q penetrates the fibrous structure and the lumens leather as a whole,

of the hides, preliminary to its tanning action thereof.

4. The method of tanning hides which comprises applying the tanning liquor of claim 2 to the hides. in pickled and drained condition, and initially plumping the hidesby reason of contact with said liquor whereby the tanning liquor penetrates the hides preliminary to the tanning action.

AM B. CO 

